1. Field of the Invention
The invention relates generally to the field of microelectronic read-only memory (ROM). More particularly, the invention relates to the field of multilevel Mask ROM.
2. Discussion of the Related Art
Conventional Mask ROMs have recently been multilevel programmed. Multilevel programming permits more data to be stored per memory cell by defining more than two (e.g., 4) memory cell states. Each of these states is defined by a different voltage threshold, which is a function of the amount of dopant implanted into the channel of a given memory cell during mask programming. The difference in multi-level thresholds may be less than one volt. Such a sub-volt difference in multi-level thresholds can be necessitated by the operating voltage of the Mask ROM (e.g., 3 V).
A problem with this technology has been that the sub-volt threshold difference tends to cause read errors. The narrow difference between voltage thresholds requires a sensitive system that is, in-turn, sensitive to even small current leakages. Current leakage can cause the associated sense amplifier to report an incorrect value for the data being read from a memory cell of interest. Therefore, what is required is a solution that reduces multi-level Mask ROM read errors by controlling current leakage.